The wall is located at a geographic narrowing between the Caspian Sea and the mountains of northeastern Iran. It is one of several Caspian Gates at the eastern part of a region known in antiquity as Hyrcania, on the nomadic route from the northern steppes to the Iranian heartland. The wall is believed to have protected the Sassanian Empire to the south from the peoples to the north,[2] probably the White Huns. However, in his book Empires and Walls, Chaichian (2014) questions the validity of this interpretation using historical evidence of potential political-military threats in the region as well as the economic geography of Gorgan Wall's environs.[3] It is described as "amongst the most ambitious and sophisticated frontier walls" ever built in the world,[4] and the most important of the Sassanian defense fortifications.[5]

It is 195 km (121 mi) long and 6–10 m (20–33 ft) wide,[6] and features over 30 fortresses spaced at intervals of between 10 and 50 km (6.2 and 31.1 mi). It is surpassed only by the walls systems of Great Wall of China as the longest single-segment building and the longest defensive wall in existence.

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Among archaeologists the wall is also known as "The Red Snake" because of the colour of its bricks. In Persian, it was popularized by the name "Alexander Barrier" (سد اسکندر‎ Sadd-i-Iskandar) or "Alexander's Wall", as Alexander the Great is thought by early Muslims to have passed through the Caspian Gates on his hasty march to Hyrcania and the east. It is also known as the "Anushirvân Barrier" (سد انوشیروان‎) and "Firuz/Piruz Barrier" (سد پیروز‎), and is officially referred to as "Gorgan Defence Wall" (دیوار دفاعی گرگان‎). It is known as Qïzïl Yïlan or Qazal Al'an to local Iranian Turkmens.[1]

The barrier consists of a wall, 195 km (121 mi) long and 6–10 m (20–33 ft) wide,[6] with over 30 fortresses at intervals of between 10 and 50 km (6.2 and 31.1 mi).[6][7]

The building materials consist of mud-brick, fired brick, gypsum, and mortar. Clay was also used during the early Parthian period. Mud-bricks were more popular in the early period in the construction of forts and cities, while fired bricks became popular in the later period. Sometimes one brick was set in the vertical position, with two horizontal rows of bricks laid above and below. The sizes of mud or fired bricks differ, but in general the standard size was 40 × 40 × 10 cm.[1] The fired bricks were made from the local loess soil, and fired in kilns along the line of the wall.[6]

This wall starts from the Caspian coast, circles north of Gonbade Kavous, continues towards the northeast, and vanishes in the Pishkamar Mountains.
The wall lies slightly to the north of a local river, and features a 5 m (16 ft) ditch that conducted water along most of the wall.[2]

In 1999 a logistical archaeological survey was conducted regarding the wall due to problems in development projects, especially during construction of the Golestan Dam, which irrigates all the areas covered by the wall. At the point of the connection of the wall and the drainage canal from the dam, architects discovered the remains of the Great Wall of Gorgan. The 40 identified castles vary in dimension and shape but the majority are square fortresses, made of the same brickwork as the wall itself and at the same period.[2] Due to many difficulties in development and agricultural projects, archaeologists have been assigned to mark the boundary of the historical find by laying cement blocks.

Larger than Hadrian's Wall and the Antonine Wall taken together (two separate structures in Britain that marked the northern limits of the Roman Empire), it has been called the greatest monument of its kind between Europe and China. The wall is second only to the walls that make up the Great Wall of China as the longest defensive wall in existence, and although now in substantial disrepair, it was perhaps even more solidly built than the early forms of the Great Wall.[2]

The route, from east to west, is represented by the coordinates of the remains of the following forts and other features which lie along the wall. The coordinates and fort numbers, etc. are from Wikimapia:

If we assumed that the forts were occupied as densely as those on Hadrian's Wall, then the garrison on the Gorgan Wall would have been in the order of 30,000 men. Models, taking into account the size and room number of the barrack blocks in the Gorgan Wall forts and likely occupation density, produce figures between 15,000 and 36,000 soldiers. Even the lowest estimate suggests a strong and powerful army, all the more remarkable as our investigations focused just on 200km of vulnerable frontier, a small fraction of the thousands of kilometres of borders of one of the ancient world's largest empires.[2]

Derbent and its Caspian Gates are at the western part of the historical region of Hyrcania. While the fortification and walls on the east side of the Caspian Sea remained unknown to the Graeco-Roman historians, the western half of the impressive "northern fortifications" in the Caucasus were well known to Classical authors.